<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    // 多态存在如下条件:1 必须有继承 2 必须有父类引用指向子类对象
    // 一个符号表示多种数据类型
    var foo
    foo = {
      name:'123'
    }
    foo = 123
    foo = '123'
  </script>
</body>
</html>